Ground-state Energy Gap Research Articles

Critical slowing down in sudden quench dynamics

We reveal a prethermal dynamical regime upon suddenly quenching to the vicinity of a quantum phase transition in the time evolution of one-dimensional spin chains. The prethermal regime is analytically found to be self-similar and its duration is governed by the ground-state energy gap. Based on analytical insights and numerical evidence, we show that this dynamical regime universally exists independently of the location of the probe site, the presence of weak interactions, or the initial state. The resulting prethermal dynamics leads to an out-of-equilibrium scaling function of the order parameter in the vicinity of the transition. Our theory suggests that sudden quench dynamics, besides probing quantum phase transitions, may give rise to a universal critical slowing down near the critical point.

Theoretical investigation of the structural, optoelectronic, and the application of waste graphene oxide/polymer nanocomposite as a photosensitizer

The negligence of used waste polymers often results in the waste of resources and constitutes serious environmental pollutants. Therefore, it becomes necessary to practically provide a means of converting these waste polymers to useful resources. For this purpose, the potential applicability of some selected waste polymers as the active photosensitizer material in dye-sensitized solar cells were exploited using density functional theory. In this research work, density function theory (DFT) is applied to investigate the interaction of graphene oxide (GO) with monomers of Polypyrrole (PPy), Poly (phenylene vinylene) (PPV), Poly(vinyl alcohol) (PVA), and Polyvinyl Pyrrolidone (PVP) polymers. The geometrical structures of the hybridized nanocomposites GO-PPy, GO-PPV, GO-PVA, and GO-PVP are fully optimized at wB97XD/6–311++G(d,p) computational method. All of the nanocomposites’ optoelectronic properties, the excitation type and the wavelengths, oscillator strengths, as well as the dominant transitions were calculated. Atoms-in-molecules (AIM) and natural bond orbital (NBO) analysis were used to analyze the strength and nature of the composites. The results of the ground state energy gap revealed that the hybridized nanocomposites are semiconducting in nature while the 3.7020 eV energy gap of the GO-PVA makes it the most stable among the various nanocomposites. The thermodynamic calculation of the various nanocomposites shows that the GO-PVA nanocomposite is highly endothermic among the various nanocomposites with free energy value of 353.71kcal mol−1. The results of the density of state (DOS) analysis show that the p-orbitals in all of the different nanocomposites had the highest density contribution to the frontier molecular orbitals, and are also found to dominate the anti-bonding states densities.

Spectroscopic and Quantum Chemical Investigations of Hypothetical m-Diacetylaminoazopyrimidine and Its Photovoltaic Properties

Abstract: Here, an attempt is made to theoretically study and predict the electronic and spectroscopic (UV-Vis and IR) and structural properties, quantum chemical descriptors and subsequent application of diacetylaminoazopyrimidine in dye-sensitized solar cells (DSSCs). Ground- and excited-state time-dependent density functional theory (TD-DFT) calculations were carried out using material studio and ORCA software, respectively. The computed ground-state energy gap, chemical hardness, chemical softness, chemical potential, electronegativity and electrophilicity index are: 3.60 eV, 1.80 eV, 0.56 eV, 4.49 eV, -4.49 eV and 5.68, respectively. Conversely, the DFT-predicted excited-state quantum chemical descriptors are: 1.67 eV, 0.83 eV, 1.20 eV, 4.71 eV and -4.71 eV, corresponding to the energy gap, chemical hardness, chemical softness, chemical potential and electronegativity, respectively. Furthermore, vibrational frequency calculations confirm the presence of some key functional groups (N=N, C=O, C-H) present in the dye molecules. The computed optoelectronic parameters, such as light-harvesting efficiency, electron injection and open-circuit voltage are 0.06 eV, -8.59 eV and -5.75 eV, respectively. Overall, the dye possesses a relatively good current conversion efficiency as compared to other dyes studied in the literature; hence, it could be used as a novel material for photovoltaic technological applications. Keywords: Diacetylaminoazopyrimidine, DFT, Excited state, Spectroscopy, DSSCs.

Phase diagram of the extended Bose–Hubbard model

By means of the density matrix renormalization group technique, we accurately determine the zero-temperature phase diagram of the one-dimensional extended Bose–Hubbard model with on-site and nearest-neighbor interactions. We analyze the scaling of the charge and of the neutral ground-state energy gaps, as well as of various order parameters. In this way we come to an accurate location of the boundaries between the superfluid and the insulating phases. In this last region, we are able to distinguish between the conventional Mott insulating and density-wave phases and the Haldane insulator phase displaying long-range string ordering, as originally predicted by Dalla Torre et al (2006 Phys. Rev. Lett.97 260401).

Extended interface states enhance valley splitting inSi/SiO2

Interface disorder and its effect on the valley degeneracy of the conduction band edge remains among the greatest theoretical challenges for understanding the operation of spin qubits in silicon. Here, we investigate a counterintuitive effect occurring at Si/SiO2 interfaces. By applying tight binding methods, we show that intrinsic interface states can hybridize with conventional valley states, leading to a large ground state energy gap. The effects of hybridization have not previously been explored in details for valley splitting. We find that valley splitting is enhanced in the presence of disordered chemical bonds, in agreement with recent experiments.

Quadratic fermionic interactions yield Hamiltonians with large ground-state energy gaps

Polynomially large ground-state energy gaps are rare in many-body quantum systems, but useful in quantum information and an interesting feature of the one-dimensional quantum Ising model. We show analytically that the gap is generically polynomially large not just for the quantum Ising model, but for one-, two-, and three-dimensional interaction lattices and Hamiltonians with certain random interactions. We extend the analysis to Hamiltonian evolutions and we use the Jordan-Wigner transformation and a related transformation for spin-3/2 particles to show that our results can be restated using spin operators in a surprisingly simple manner. These results also yield a new perspective on the one-dimensional cluster state.

Theoretical study on thermal stability and absorption wavelengths of closed-ring isomers of diarylethene derivatives

Quantum chemical investigations on diarylethene derivatives with various substituents linked to different positions of thienyl rings have been carried out. The calculations on molecular structures show that the distance between two reactive carbons, which has close relationship with the photochromic reactivity and cyclization quantum yields, is prominently affected by different alkyl and alkoxy groups at 2- and 2′-positions. The ground-state energy gaps (Δ E) between closed-ring and open-ring isomers change in parallel with the aromatic stabilization energy (Δ E′) for the same series of substituents at reactive carbons. Once Δ E or Δ E′ becomes smaller, the thermal cycloreversion energy barrier becomes larger and closed-ring isomers would exhibit excellently thermal stability. The absorption wavelengths of closed-ring isomers have also been investigated using time-dependent density functional theory (TD-B3LYP/6-31G(d) or TD-PBE0/6-31G(d)). Theoretical calculations can well reproduce experimental absorption spectrum data. Furthermore, this work rationally predicts absorption wavelengths of some unsynthesized diarylethenes with various groups at 2- and 2′- or 4- and 4′-positions. The theoretical study would provide some basic insight for the novel photochromic molecular design.

Relative stabilities of the two isomers of the methanol-water dimer: The effects of the internal rotations of the hydroxyl and methyl groups of methanol

The ground-state energies of the two isomers of the methanol-water dimer, with water-donor (WM) and methanol-donor (MW) structures, have been calculated using the diffusion Monte Carlo (DMC) method with constraint dynamics. Unlike the rigid-body DMC, this method permits the internal rotations of the hydroxyl and methyl groups of methanol about the C–O bond. The DMC calculations were performed for the isotopomers CH3OH⋯H2O, CH3OD⋯H2O, and CD3OH⋯H2O. The calculations with the internal rotation of the methyl and hydroxyl groups of methanol included resulted in a much larger ground-state energy gap between the WM and MW isomers than those in which these internal rotations were frozen. This result demonstrated the critical importance of including the internal hydroxyl and methyl rotations in the DMC calculations aimed at predicting accurately the relative stabilities of the two isomers of the methanol-water dimer.

Ultrafast Dielectric Response of Proteins from Dynamics Stokes Shifting of Coumarin in Calmodulin

The contributions of protein motions to the dielectric response in solvent-inaccessible regions of calmodulin are examined. Two-pulse and three-pulse stimulated-emission experiments are used to examine the Stokes shift dynamics of coumarins in their ground and excited states. The results allow an evaluation of linear response theory. For coumarin 153, the ground- and excited-state solvation dynamics are similar to those for acetonitrile and methanol solvents. The solvation of coumarin 343 peptide in calmodulin is noticeably different in ground and excited states, indicating that the linear response picture is not appropriate in this case. The major difference is a more prominent fast component of solvation in the ground-state solvation dynamics. The ground-state energy gap correlation function is conjectured to be the best representation of protein relaxation, while that for the excited state seems to be influenced by the coupling of the protein modes to the large dipole created by excitation.

High field heat capacities of some singlet ground state systems at very low temperatures

Several aspects of high field heat capacity observed in different types of singlet ground state system are discussed. The ground state energy gap and its magnetic field dependence are also studied in the Haldane gap compounds NENP, NINO and TMNIN. The origin of the observed minimum gap in applied magnetic field is discussed. In the spin-Peierls compound CuGeO 3, the energy gap of the Cu 2+ spin system in the dimerized phase and the gapless state in the high-field magnetic phase are confirmed.

Peierls Instability and Charge-Density-Wave Transport in Fluoranthene and Perylene Radical Cation Salts

Fluoranthene and perylene radical cation salts are quasi-one-dimensional conductors, which show a Peierls transition to a charge-density-wave ground state. Radical cation salts with different stoichiometries composed of the aromatic donor molecules fluoranthene (Fa = C16H10) or perylene (Per = C20H12) and the counterion X — = PF fi were investigated. We find that the temperature dependence of the electrical conductivity and the properties in the charge-density-wave ground state strongly depend on the composition of the crystals. When fluoranthene is replaced by perylene, the Peierls transition is shifted from 180 K to 120 K and the ground state energy gap decreases from values of 120-180 meV to 60-80 meV. In the charge-density-wave ground state the threshold field for nonlinear conductivity varies between 0.1 and 1 V/cm for different systems. Our results indicate that disorder, three-dimensional coupling and commensurability effects play an important role for the transport properties of these systems.

Experimental tests of fractional quantum hall effect theory

The predicted effect of disorder on the hierarchical model of the FQHE, the extension of the FQHE to the N=1 Landau level and the Laughlin scaling diagram are examined in a range of experiments. A new method of obtaining the ground state energy gaps by analysis of the widths of fractional resistivity minima is outlined and preliminary data are presented that point to a method for the experimental determination of quasi-particle charge.